CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020
Cash flows from operating activities
Net loss	$ (93,101)	$ (74,496)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	19,663	11,467
Reduction in related party contract asset due to Amended Collaboration Agreement with Merck	4,600	0
Depreciation	4,610	5,002
Amortization of premium (discount) on marketable securities	2,546	(208)
Other non-cash expenses	29	236
Changes in operating assets and liabilities:
Related party receivable from collaboration	(4,067)	(2,009)
Related party contract asset	1,500	0
Prepaid expenses and other assets	(3,649)	(3,715)
Accounts payable	(2,031)	(7,108)
Accrued liabilities	4,931	9,765
Deferred rent	(2,213)	(2,104)
Contract liabilities	12,288	(286)
Net cash used in operating activities	(54,894)	(63,456)
Cash flows from investing activities
Purchase of marketable securities	(194,525)	(29,399)
Proceeds from maturities of marketable securities	102,500	94,537
Net purchase of property and equipment	(1,551)	(1,605)
Net cash (used in) provided by investing activities	(93,576)	63,533
Cash flows from financing activities
Proceeds from follow on offering, net	134,580	0
Proceeds from exercise of stock options	11,094	7,214
Proceeds from employee stock purchase plan	1,409	1,285
Proceeds from Open Market Agreement	196	0
Deferred offering costs paid	0	(573)
Net cash provided by financing activities	147,279	7,926
Net (decrease) increase in cash and cash equivalents	(1,191)	8,003
Cash, cash equivalents and restricted cash, at beginning of period	148,516	247,472
Cash, cash equivalents and restricted cash, at end of period	147,325	255,475
Non-cash investing and financing activities:
Vesting of common stock from early exercises	45	405
Property and equipment purchases accrued and not yet paid	35	0
Deferred offering costs accrued but not yet paid	$ 0	$ 21